1. Field of the Invention
The present invention is broadly concerned with vaccines composed of recombinant bovine herpesviruses and corresponding methods. More particularly, in its preferred form, the present invention is concerned with the construction of an infectious recombinant bovine herpesvirus type 1 having a functional .beta.-galactosidase gene inserted in and thereby inactivating the thymidine kinase gene (e.g., Gal-TK); infection of an animal with the resultant avirulent vaccine virus is detected by assaying the animal's respiratory secretions for the presence of virus having .beta.-galactosidase activity in host cells.
2. Description of the Prior Art
Bovine herpesvirus type 1 (BHV-1), also known as infectious rhinotracheitis virus (IBR), is associated with a variety of clinical diseases including rhinotracheitis, conjunctivitis, genital infections, abortion, enteritis, encephalitis, and generalized systemic infections in cattle (Ludwig, 1983; Wyler et al., 1989). The genome of BHV-1 consists of a linear double-stranded DNA molecule about 140-kb in length and is composed of a unique long (U.sub.L) region and a unique short (U.sub.S) region. The U.sub.s region is flanked by an internal and a terminal inverted repeat sequence (I.sub.R and T.sub.R, respectively). The BHV-1 genome encodes approximately 70 proteins (Misra et al., 1981). Many herpesviruses, including BHV-1, encode thymidine kinase (TK) (Kit and Qavi, 1983; Weinmaster et al., 1982). The BHV-1 TK gene has been mapped (Bello et al., 1987), and the sequences of the TK genes of four BHV-1 strains have been determined. (Kit and Kit, 1987; Mittal and Field, 1989; Smith et al., 1990; Bello et al., 1992).
The TK genes of these four strains encode between 357 and 359 amino acid (aa) residues. The deduced aa sequences of these genes reveal that BHV-1 TK is composed of five protein domains (I-V). These domains have a high degree of homology to corresponding domains in TKs of other herpesviruses, suggesting that these domains have functional significance.
The TK gene is not essential for replication of BHV-1 in vitro or in vivo (Kit and Kit, 1987; Mittal and Field, 1989), but does play an important role in pathogenicity. BHV-1 and other herpesviruses having defective TK genes grow normally in vitro but are highly attenuated in vivo (Field and Wildy, 1978; Kit et al., 1985 a, b; Slater et al., 1993). Thus, the basis of several BHV-1 vaccines has been inactivation of the TK gene (Kit and Qavi, 1983; Kit and Kit, 1987; Kit and Kit, 1989).